The present invention relates to rolling-traction variators of the type in which drive is transmitted from one race to another by one or more rollers whose orientation is variable in accordance with changes in variator drive ratio. More particularly, the invention concerns a novel mechanism for control of roller orientation.
The word “variator” as used herein refers to a transmission device which provides a continuously variable ratio. FIG. 1 illustrates, purely by way of example rather than limitation and in highly simplified form, some of the principal components of a known rolling-traction type variator 10 in which drive is transmitted from outer discoidal races 12, 14 to an inner discoidal race 16 (or vice versa) through rollers 18 running upon the races. Only two rollers are shown but a practical variator typically has six such rollers in total, with three in both of the cavities 38 defined between the races. Traction between the rollers and races is provided by biasing them toward each other, which is achieved in this example by means of a hydraulic actuator 20 urging one race 14 toward the others. In the illustrated example the left hand outer race 14 is keyed to a variator shaft 22 to rotate along with it, while the right hand outer race 12 is in this illustration integrally formed with the shaft. Inner race 16 is journalled for rotation about the shaft, which may be driven from an engine schematically represented at 23. Rotation of the outer races 12, 14 turns the rollers 18 and hence also the inner race 16. Power take-off from the inner race may be achieved by a chain running upon it, or through some co-axial arrangement, as is well known in the art.
The rollers are able to “precess”. That is, each can change its orientation, varying the inclination of the axis of the roller to the “variator axis” 21 defined by the shaft 22. Two alternative orientations of the rollers 18 are respectively indicated in solid lines and in phantom in FIG. 1. It will be apparent that by moving from one orientation to another each roller changes the relative circumferences of the paths it traces out upon the inner and outer races, thereby enabling a change in the variator drive ratio.
Hence the roller's mountings must enable it both to spin about its own axis and also to turn about a different axis which will be referred to as the “precession axis”. Correspondingly the turning motion which changes roller orientation will often be referred to as “precession” herein. Roller precession is not controlled directly, by applying torque to the roller mountings about the precession axis. Instead, the roller's mountings leave the roller free to precess and roller orientation is controlled by virtue of a steering effect exerted upon the rollers by the races. As an example of this, consider the known variator construction illustrated in FIGS. 2 and 3. These drawings are taken (with some modifications) from Torotrak (Development) Limited's patent GB 2227287 and for more detail on the construction and operation of this and other types of variator, reference should be made to that document. FIGS. 2 and 3 show only two of the variator's races 12, 16. Each roller 18 (only one of which is shown) is journalled in a movable carrier 30, which is coupled to a piston 32 running in a cylinder 34. The roller and its carrier are able to precess together about a precession axis 36 determined, in this particular construction, by the positioning of the cylinder 34. Note that the precession axis does not lie in a radial plane. Instead it forms a “castor angle” CA with the radial plane, as seen in FIG. 2. As the piston moves back and forth along the cylinder, the roller likewise moves back and forth. The races 12, 16 are in this example shaped to define a toroidal cavity, containing the rollers, of circular section, similar to the cavities 38 seen in FIG. 1. The races constrain the roller 18 so that as it moves back and forth its centre follows a path which is an arc of the centre circle 40 of the torus. This centre circle is the locus of the centre points of the generator circles of the torus. Movement of the roller along this path depends upon the balance between (a) the circumferential component 2F of a biasing force applied to the roller's carriage by the piston 32 and (b) the two forces F exerted upon the roller 18 by the respective races 12, 16.
The rollers each tend toward a position in which, at the “contacts” between the roller 18 and the races 12, 16 (the word “contacts” is used in a loose sense because these components do not actually touch, being separated by a thin film of traction fluid, as known in the art) the motion of the roller periphery is parallel to the motion of the surface of the race. A mismatch between roller and race movement at the contacts results in a steering moment upon the roller about the precession axis, tending to cause the roller to precess to reduce the mismatch. The condition for the two movements to be parallel (i.e. for zero steering moment) is that the axis of the roller should intersect the variator axis.
Consider what happens as the roller/carriage assembly 18, 30 is displaced to the left or right in FIG. 3. If the roller axis 41 initially intersects the variator axis 21, the roller's displacement takes it away from such intersection but only transiently, because the resulting steering moment causes the roller to precess as it is displaced. By virtue of the castor angle CA, such precession is able to restore the intersection of the two axes. The result is that the roller's “precession angle” is a function of its displacement along its circular path 40. In this known construction the relationship between roller position and precession angle depends upon the castor angle.
The arrangement offers the facility for the variator to be “torque controlled”. This manner of variator operation has been explained in various published patents in the Torotrak (Development) Limited name including European Patent 444086 and is known to those skilled in the art. To briefly explain, in a torque controlled variator the variator ratio is not directly controlled. A controlled biasing force (the force 2F in FIG. 2) is applied to each of the rollers and at equilibrium this must be balanced by the forces exerted upon the roller by the variator races (the forces F in FIG. 2). The forces exerted by the races upon the rollers are determined by the torques upon the respective variator races as well as the radii of the paths traced upon the discs by the rollers. A simple analysis shows that:Actuator Biasing Force α Tin+Tout where Tin and Tout are the torques upon the inner and outer variator discs, respectively. The sum Tin+Tout is referred to as the “reaction torque” and it is this quantity, rather than variator ratio, which is directly controlled. Changes in variator ratio result from the application of Tin and Tout (added to externally applied torques, e.g. from a driving engine) to the inertias acting on the variator's input and output. The rollers automatically move and precess in accordance with consequent changes in variator ratio.
Another type of known variator construction is found for example in GB 1002479 and is illustrated in FIG. 4. Variator rollers are again indicated at 18, although here a full set of three rollers in one cavity is shown, and are journalled on bearings 50 in carriers 52 at opposite ends of which are spigots 54, 56 received in aligned bores in a spider structure 58. The carrier is thus able to move slightly back and forth along a direction transverse to the variator axis. Such carrier movement is controlled by a three-spoked thrust receiving member 60 coupled to each carrier by a respective ball and socket joint 62. Slight rotational motion of the member 60 about the variator axis causes the rollers and carriers to move along the aforementioned transverse direction. The aligned bores receiving the spigots 54, 56 can be offset along the axial direction to create a castor angle and the steering effect explained above is used to control roller orientation. The roller's bearings 50 allow it some lateral “float” so that it can follow the necessary circular path about the variator axis, despite the carrier 52 following a straight line.
The above embodiments involve the carrier and roller rotating together to achieve the required roller precession. A different approach to roller control is taught in Torotrak (Development) Limited's international patent application PCT/GB03/00259, published under WO 03/062670, and FIG. 5 shows the relevant arrangement. Here, the carrier 70 has twin piston heads 72, 74 at its opposite ends which run in respective cylinders 76, 78. Each variator roller, a single example of which is once more indicated at 18, can spin about its own axis by virtue of a roller bearing 80, but can also precess relative to the carriage because the bearing 80 is coupled to the carriage through a gimbal arrangement comprising a ball 82 and a spline 84, the spline defining the axis about which the roller precesses with respect to the carriage. Here the carrier itself is unable to rotate because the centre of the roller is offset from the axis of the piston heads 72, 74. Among the advantages of this arrangement is the fact that the castor angle, being defined by the positioning of the spline 84, can be freely chosen. In the earlier described variators problems e.g. of fouling with the variator discs constrain the choice of castor angle.
Nonetheless all of the above described variators have it in common that the steering effect required to control roller orientation is achieved simply by displacing the carrier back and forth along the centre circle of the toroidal cavity.
It is an object of the present invention to provide improvements in the manner of control of the roller(s) in a rolling-traction type variator.